Coagulation Cascade, Tests for Coagulopathy, and ASRA Guidelines (Latest Edition)

PART 1 - THE COAGULATION CASCADE [8 marks]

Overview

A. Extrinsic Pathway (Tissue Factor Pathway)

• Initiated when vascular injury exposes Tissue Factor (TF) on subendothelial cells
• TF binds circulating factor VII -> forms TF-VIIa complex (extrinsic tenase)
• TF-VIIa activates factor X -> Xa (directly) and factor IX -> IXa (cross-activation to intrinsic pathway)
• This is the primary initiator of coagulation in vivo
• Measured in vitro by the Prothrombin Time (PT) assay

B. Intrinsic Pathway (Contact Activation Pathway)

• Initiated by contact with negatively charged surfaces (collagen, glass)
• Factor XII (Hageman factor) is activated -> XIIa
• XIIa activates XI -> XIa
• XIa activates IX -> IXa
• Thrombin (generated from the extrinsic pathway) also cleaves XI -> XIa (amplification loop)
• IXa + VIIIa + phospholipid + Ca²+ = intrinsic tenase complex -> activates X -> Xa
• Factor VIII circulates bound to von Willebrand factor (vWF); vWF deficiency secondarily causes low factor VIII
• Measured by the aPTT (Activated Partial Thromboplastin Time)

Note: Factor XI deficiency causes only mild bleeding, suggesting the contact pathway plays a minor role in hemostasis in vivo; factor XI appears more relevant in pathologic thrombosis. - Barash Clinical Anesthesia 9e

C. Common Pathway

• Factor Xa (from either pathway) combines with factor Va + phospholipid + Ca²+ = prothrombinase complex
• Prothrombinase converts Prothrombin (factor II) -> Thrombin (IIa)
• Thrombin cleaves fibrinogen -> fibrin monomers, which polymerize
• Factor XIIIa (activated by thrombin) cross-links fibrin polymers -> stable insoluble fibrin clot
• Fibrin also cross-links activated platelets via GP IIb/IIIa receptors

D. Thrombin - Central Role

1. Activates platelets via PAR-1 and PAR-4 receptors
2. Activates factor V -> Va, factor VIII -> VIIIa, factor XI -> XIa (intrinsic amplification)
3. Activates factor XIII -> XIIIa (clot stabilization)
4. Cleaves fibrinogen -> fibrin

E. Vitamin K-Dependent Factors

F. Natural Anticoagulant Systems

1. Antithrombin III (AT-III) - inhibits thrombin and factor Xa; activity enhanced 1000x by heparin
2. Protein C / Protein S system - thrombomodulin + thrombin activates Protein C -> degrades Va and VIIIa
3. Tissue Factor Pathway Inhibitor (TFPI) - inhibits TF-VIIa-Xa complex

PART 2 - TESTS FOR COAGULOPATHY [6 marks]

Standard Laboratory Tests

Interpretation of Coagulation Test Abnormalities

Point-of-Care Viscoelastic Testing

1. TEG (Thromboelastography) - Parameters:
   • R (reaction time) - time to initial fibrin formation
   • K - clot formation kinetics
   • α-angle - rate of clot strengthening
   • MA (Maximum Amplitude) - clot strength (platelet + fibrin contribution)
   • LY30 - percentage lysis at 30 minutes
2. ROTEM (Rotational Thromboelastometry) - Similar metrics:
   • CT (clot time), CFT (clot formation time), MCF (maximum clot firmness), CLI30
   • Multiple activators: EXTEM (extrinsic), INTEM (intrinsic), FIBTEM (fibrinogen), HEPTEM (heparin effect)

TEG and ROTEM provide a more complete in vivo assessment than standard assays - they evaluate primary and secondary hemostatic pathways together, detect hyperfibrinolysis, and guide targeted blood product therapy. An INR of 3 with platelet count of 40,000/µL may paradoxically represent a hypercoagulable state in cirrhosis. - Morgan & Mikhail's Clinical Anesthesiology 7e

Additional Specific Tests

• Anti-Xa level - monitors LMWH activity (therapeutic range 0.5-1.0 IU/mL)
• ACT (Activated Clotting Time) - monitors high-dose heparin (cardiopulmonary bypass; target >480 sec)
• dTT (dilute Thrombin Time) / ECT (Ecarin Clotting Time) - monitor direct thrombin inhibitors (dabigatran)
• Mixing studies - PT/aPTT corrects with normal plasma in factor deficiency; fails to correct in the presence of inhibitors (e.g., lupus anticoagulant)

PART 3 - ASRA GUIDELINES (Latest - 5th Edition, 2025) [6 marks]

Background

• Risk of spinal hematoma must be weighed against benefit of regional anesthesia for each individual
• Timing of needle/catheter insertion and removal must reflect the pharmacokinetics of the specific anticoagulant
• Frequent neurologic monitoring is essential
• Concurrent use of multiple anticoagulants increases hemorrhagic risk
• These guidelines do NOT define standard of care and do not replace clinical judgment

Drug-Specific Recommendations (ASRA 4th + 5th Edition)

1. Unfractionated Heparin (UFH)

Heparin can be given intraoperatively after neuraxial needle placement (after 1 hour interval); this sequence is used safely in vascular surgery.

2. Low Molecular Weight Heparin (LMWH)

3. Warfarin (VKA)

• Stop warfarin 5 or more days before surgery
• Check INR within 24 hours pre-procedure
• Neuraxial block only if INR is normal (≤1.1 per 5th edition; ≤1.5 per 4th edition)
• Remove catheter when INR ≤1.5 (with monitoring for neurologic changes)
• Single dose of warfarin ≤24 hours before surgery: neuraxial block permissible (4th edition)
• Low-dose oral Vitamin K (1-5 mg) can be given if INR 1.5-3.0 at 6-10 hours pre-procedure

4. Direct Oral Anticoagulants (DOACs)

For therapeutic (high-dose) DOACs: 72-hour drug-free interval is recommended. Laboratory confirmation (anti-Xa or dTT) is advised if CrCl <30 mL/min. - ASRA 5th Edition 2025

5. Antiplatelet Agents

Neuraxial catheters should NOT be maintained with prasugrel or ticagrelor due to rapid onset and offset. With clopidogrel/ticlopidine (no loading dose), catheter may remain 1-2 days. - ASRA 4th Edition; OpenAnesthesia

6. Fibrinolytics / Thrombolytics (e.g., tPA, streptokinase)

• Absolute contraindication to neuraxial procedures within 10 days of thrombolytic use
• If catheter in situ when thrombolytic given: avoid removal until fibrinogen level normalizes (>100 mg/dL)
• Monitor neurologic status every 2 hours

7. Deep Peripheral and Plexus Blocks

• The 4th and 5th editions explicitly apply neuraxial recommendations to deep peripheral nerve and deep plexus blocks (e.g., lumbar plexus, paravertebral, deep cervical plexus)
• Superficial nerve blocks may be performed without therapy-free intervals regardless of anticoagulant dose (low bleeding risk)

Signs of Spinal Hematoma (Warning Signs - Monitor Post-Neuraxial)

• New or worsening back pain
• Bowel or bladder dysfunction
• Progressive motor weakness or sensory deficit after apparent resolution
• If suspected: urgent MRI and neurosurgical consultation - outcomes best if decompressed within 8 hours of onset

Summary Quick-Reference Table

• Coagulation cascade: Robbins Pathologic Basis of Disease (Robbins), p.122; Barash Clinical Anesthesia 9e, pp. 1268-1278
• Tests for coagulopathy: Morgan & Mikhail's Clinical Anesthesiology 7e, Table 34-1; Barash Clinical Anesthesia 9e, p.1278
• ASRA Guidelines: Morgan & Mikhail's Clinical Anesthesiology 7e, p.1505 (4th Edition); ASRA 5th Edition, RAPM 2025; Miller's Anesthesia 10e, p.11743

ASRA Guidelines - Complete Table: Needle/Catheter Insertion AND Catheter Removal Timing

The risk of spinal hematoma is equally significant at the time of catheter removal as at needle insertion, because removal disrupts the epidural venous plexus in an anticoagulated patient.

1. Unfractionated Heparin (UFH)

2. Low Molecular Weight Heparin (LMWH)

Anti-Xa level measurement is recommended before removal if renal impairment (CrCl <30 mL/min) is present.

3. Warfarin (VKA)

The INR should be checked daily while catheter is in situ in patients restarted on warfarin. If INR rises to 1.5-3.0, increase monitoring; if INR >3.0, hold warfarin and do not remove catheter until INR falls.

4. Direct Oral Anticoagulants (DOACs)

For all DOACs in patients with CrCl <30 mL/min (or dabigatran with CrCl <50 mL/min): extend drug-free intervals and confirm with laboratory testing (anti-Xa or dTT) before catheter removal.

5. Antiplatelet Agents

6. Fibrinolytics / Thrombolytics

7. Herbal Supplements (new in 4th/5th editions)

• Garlic, Ginkgo, Ginseng - no mandatory stop; use caution with concurrent anticoagulants
• No specific catheter removal interval required for herbal agents alone

Complete Summary Quick-Reference

The interval before catheter removal = the same interval required before needle insertion for most drugs. The interval before resuming the drug after removal is generally shorter (1-6 hours), to allow the puncture site to seal before re-anticoagulating.